Bolt Carrier Group

The bolt carrier group (BCG) is the beating heart of the AR-15 platform — responsible for chambering, firing, extracting, and cycling each round. It is a critical component of the gas system and a critical component of the reciprocating mass. Yet, despite its critical role, the BCG is often overlooked or misunderstood during the build process. With dozens of finishes, materials, and design variations on the market, choosing the right BCG can be confusing — even for experienced builders.

This guide is designed to cut through the noise. Rather than focusing on brand names or marketing hype, it breaks down the functional differences in materials, coatings, tolerances, and component design. Whether you’re building a precision rifle, a hard-use duty carbine, or a general-purpose AR, understanding how each BCG attribute affects performance will help you make informed, mission-appropriate decisions.

By the end of this article, you’ll be equipped to confidently select a bolt carrier group that matches your specific performance goals, environmental demands, and maintenance preferences.

Disclaimer: An Industry of Fakers

The BCG is a complex machine, composed of no less than 15 sub-components. The Colt Technical Data Package (TDP) details the requirements for each of them, from materials, to dimensions, to treatments, to finishes. These specifications ensure compatibility, efficiency, durability, and performance of the system.

Unfortunately, many civilian consumer manufacturers don’t commit to the TDP, even though most throw around the term “Mil-Spec” as if saying it makes it true. Many components are made from inferior materials and are dimensionally non-conformant. Many are not assembled properly. Many are not tested or inspected properly. For these reasons, we trust very few BCG and BCG sub-component manufacturers. In fact, we were so dissatisfied with the options, that we designed our own line of BCGs.

TDP Specifications for the BCG

The Colt TDP defines the specifications for the BCG and its sub-components. You can find the relevant TDP drawings in our Spec to Inspect series in Para Bellum University.

The following table lists the TDP specified materials and finishes. In the absence of a well-researched understanding of alternatives, you should always consider the TDP to be the North Star for specifications. Some alternatives are equivalent to the TDP spec and some are an improvement. But, some alternatives are inferior and you should be aware of these.

Design Specifications for the AR Bolt Carrier

AR Bolt Carrier Materials

The TDP standard for carrier material is 8620 steel. We would not stray away from 8620 in the steel family. There really isn’t an excuse. 8620 is not expensive, is readily available, and is easy to machine.

Some manufacturers offer carriers made of lightweight materials. Carriers made of lightweight materials are designed for competition, where the lower inertia means earlier and faster cycling, which gives microseconds of advantage. However, these carriers are sometimes used as a band aid for under-gassed guns. We strongly discourage you from using a lightweight carrier unless you are shooting in carbine competitions or are firing a low pressure round (like sub-sonic .300 Blackout) and can’t get the gun to work any other way. If your gun is under-gassed, it is not because your carrier is too heavy.

Carriers milled from a lightweight material are made from either aluminum or titanium. Carriers milled from aluminum or titanium are less durable than steel carriers. You can expect a much shorter life from a carrier made from these materials. Aluminum, in particular, should only be used for competition guns and should be considered consumable. Be sure to check these carriers for early signs of failure before they blow up in your gun.

For more information on the metals used in the BCG, check out our Design article on Metals and Finishes.

AR Bolt Carrier Finishes

The TDP standard for carrier finish is manganese phosphate with a chrome bore. There is nothing wrong with phosphate and the properties of chrome are very important to the bore.

There are a a few finishes that offer a dramatic improvement over phosphate.

Nitride is a common alternative. We aren’t huge fans of nitride carriers for a direct impingement AR. The bore of the carrier really benefits from the hard chrome plating and a nitride carrier is nitride all over; it cannot be chrome plated. Chrome is harder, slicker, more corrosion resistant, has superior wear resistance, and has better thermal tolerance over nitride.

Chrome is a great upgrade to the finish of the carrier. Hard chrome inside and out offers substantial improvements in hardness, lubricity, and wear resistance. Dollar-for-dollar, this is the best bang for your buck.

NP3 is a great finish, when done right. We don’t use it, but its cool if you are willing to pay for it.

We are not big fans of nickel boron (NiB) or titanium nitride (TiN) for the carrier. There are many reasons; we’ll get into them eventually, but not here.

Diamond Like Carbon (DLC) is a phenomenal coating, when done right. It is super hard and super slick. The finish has to be expertly applied to the substrate to avoid flaking off. But when done right, this coating is amazing. There are a lot of haters of DLC. They say its just a Gucci coating and not worth the price…yet they’ll go out and pay more for NiB or TiN, which are 100% inferior to DLC. We love DLC…especially for the BCG.

For more information on the finishes used on the BCG, check out our Design article on Metals and Finishes.

AR Bolt Carrier Designs

AR-15 versus M16 Cut

There are two major carrier profiles: M16/Full Auto and AR-15/Semi Auto

The AR-15 cut is designed for civilian use and was a requirement during the Assault Weapons Ban, which expired in 2004. It is not compatible with full auto triggers because of the shorter rear profile cannot engage an auto sear. Because there is less material, an AR-15 bolt carrier is lighter than an M16 carrier. You may have a hard time finding new AR-15 BCGs, because nowadays, everyone just makes M16 BCGs.

The M16 cut is designed for full auto use. This is based on the original design of Eugene Stoner. Despite the name, there is nothing illegal about owning an M16 BCG, at least not since the AWB expired. Because of the additional material versus a semi auto carrier, an M16 bolt carrier will be a little heavier. But this is not necessarily a bad thing. M16 cut carriers are the standard now. Nearly every BCG you find on the market today features an M16 cut carrier.

In our opinion, there is no reason to ever buy an AR-15 cut bolt carrier.

Lightweight Profile

Some bolt carrier manufacturers achieve a lightweight carrier by strategically removing material from the carrier (instead of using a lighter material).

Normally, we would say removing material weakens the part and this is bad. However, if you are trying to lighten your reciprocating mass for a legitimate reason, we would rather you have a steel carrier with a lightweight profile than we would you have a lighter BCG made from a less robust material.

Our only caution with a lightweight carrier profile is to make sure you use one with forward assist serrations.

Forward Assist Serrations

Forward assist serrations are half-moon shaped cuts on the right side of the carrier. The forward assist pawl engages these cuts, allowing you to drive the BCG forward without racking the charging handle.

As a combat or defensive weapon, we do not recommend an AR without a functional forward assist. We don’t understand why people are vehemently against them. The forward assist is an important functional feature of the AR (it’s actually one of the things we disagree with Eugene Stoner on). If your bolt fails to go into battery when you need it to, you can easily push it into battery with the forward assist (instead of having to rack the charging handle). If you are trying to be quiet, you can rack the charging handle gently and push the bolt into battery. These are things you cannot do without a forward assist.

Some carriers save weight by removing material. When the material removed precludes the use of the forward assist (e.g. the JP Enterprises LMOS), you lose the functionality of your forward assist, whether or not your upper is equipped with one.

Design Specifications for the AR Bolt Assembly

The AR bolt is one of the most abused parts of the AR. It undergoes high impact, shear, tensile, and compression stresses, every time the gun cycles. To ensure a reliable and durable bolt with a long life, material and finish is paramount.

AR Bolt Materials

The TDP for the bolt calls for Carpenter 158 steel (C158). This is a very specific, proprietary composition only manufactured by one company: Carpenter Technology.

There is one suitable alternative, if it is manufactured correctly (which is a big “if”). That alternative is 9310 steel. Where C158 is only manufactured by one company, 9310 can be made by anyone. With the proper heat treatment, 9310 is comparable (if not slightly better by some measures) than C158 steel. However, given the “if”, the government has stuck with C158. We have done the same for our AR-15 BCGs.

In our opinion, anything other than C158 or 9310 (from a reputable manufacturer) should not be used.

S7 tool steel is NOT an acceptable alternative to C158. We will explain this in plenty of detail in an upcoming article dedicated to the S7 bolt.

For more information on the metals used in the BCG, check out our Design article on Metals and Finishes.

AR Bolt Finishes

The TDP calls for manganese phosphate for the bolt head, extractor, extractor pin, and ejector.

The extractor should ALWAYS have a phosphate finish. You do not want your extractor to be slick. It will slip off of the case rim when it tries to extract, leading to a failure-to-extract malfunction. You do not want an additive finish that affects the dimensions of the part. The tolerance for the extractor groove is very slim. If you clog this up with an applied finish, it the case rim will not fit into the groove. If the case rim doesn’t fit into the groove, the case won’t be extracted.

There is no benefit to anything other than phosphate for the extractor pin or ejector. Any finish that adds to a dimension can be an issue for both. Stick with phosphate.

The bolt head is the only part of the bolt that can benefit from an alternate finish.

Chrome is a great upgrade to the bolt finish.

NP3 is a great finish for the bolt head, if done right.

Diamond Like Carbon (DLC) is another great finish for the bolt head, if done right.

Nitride is a common alternative for the entire BCG. Nitride (especially QPQ) is an acceptable finish for the carrier. We strongly discourage the use of nitride bolts. If you want to know why, check out our Don’t Buy a Nitride Bolt article.

For more information on the finishes used on the BCG, check out our Design article on Metals and Finishes.

AR Bolt Testing and Inspection

The TDP calls for stress testing every single bolt by firing a high pressure proof round (M197). you will see this referred to as high pressure testing or HPT. This round generates about 70,000 psi (well over the normal 55,000 psi of a M193 round). This overpressure round will stress the bolt and cause fracture at any defects in the bolt.

The TDP also calls for magnetic particle inspection (MPI) after firing the proof load. This is an inspection procedure that involves application of a fluorescent magnetic suspension (wet) or powder (dry) to the bolt while a magnetic field is applied. If there are any microscopic fractures in the bolt (from the over-pressured round), the magnetic field flowing through the bolt “leak” away from the surface and cause the magnetic particles to gather near defects (even cracks below the surface). When viewed under ultraviolet light, the magnetic particles will glow and will highlight the failure points. The video below walks through the process and illustrates the way this inspection works.

These tests are very important. They ensure that each bolt is free of metallurgic defects that will cause premature failure.

Some manufacturers perform “batch” MPI testing. This means they take a representative sample of each batch of bolts and inspect them for fracture. Batch testing is not the same as testing every bolt. Bolts produced in the same batch are not identical. The forging is not the same. The machining is independent. To assume that one defect-free bolt means that all are defect-free is false. This is why the TDP calls for 100% inspection.

Other AR BCG Design Considerations

Gas Efficiency

For the direct impingement AR, gas efficiency is the root of many malfunctions and performance issues. The bolt carrier group is a critical component of the gas system.

The design of the gas system components, per the TDP, ensure efficiency. The problem is that civilian manufacturers don’t necessarily live by the TDP (they don’t have to answer to the U.S. Military when something is out of spec).

Gas efficiency relies on tight junctions between interfacing components. If the tolerances are loose, gas will escape the system before it does what it needs to. The drop in pressure and flow will reduce the efficiency of the system (i.e. doing more with less).

Poor efficiency leads to under-gassed malfunctions (failure-to-extract, failure-to-eject, failure-to-feed, short stroking, failure to lock bolt, etc.). For more information, check out our Gas System Deep Dive.

Many of these malfunctions that are tied to poor efficiency lead DIY-ers to a drawer of Band Aids, including lightweight carriers, light buffers, weak buffer springs, etc. Our advice: don’t treat the symptoms; address the problem at the root.

To ensure an efficient bolt carrier group, you need to focus on the dimensions of the sub-components of the assembly. There are a few critical dimensions that make all the difference. For more, check out our Spec to Inspect Deep Dive series. If you are interested in the specs that we use, see the Deep Dive article on PBA Specs.

We recognize that most of you have no interest in investing in thousands of dollars worth of instruments, gauges, and training to make sure keep BCG manufacturers honest. Honestly, you shouldn’t have to. The alternative is to buy your BCG from a manufacturer that commits to the TDP and has a robust quality management system that ensures conformant and efficient product. Unfortunately, there aren’t many that we can recommend:

Para Bellum Arms (check out our BCGs and the story that goes along with them)
Colt
Bravo Company Manufacturing
Sons of Liberty Gun Works

Gas Key Sealing

One of the most important, yet underappreciated and underperformed, procedures is gas key sealing.

The TDP calls for the use of Permatex Aviation Form-A-Gasket No. 3. This liquid gasket cures to form a semi-flexible seal between the bottom of the gas key and the top of the bolt carrier.

Along with correct torque and staking of the gas key screws, this seal prevents the leaking of gas as the bolt carrier pressurizes. Without this gasket, pressure will bleed off between the key and carrier, and efficiency will be impaired.

Typically, if a manufacturer seals the gas key, they disclose this fact. If they don’t say that they do it, there is a good chance that they do not.

Gas Key Staking

Staking the gas key is an important step in ensuring that it stays attached to the bolt carrier.

Staking involves deforming some material from the gas key (around the edge of the gas key screws) against the sides of the gas key screws. When performed correctly, this “locks” the gas key screws in place, which prevents them from loosening and backing out.

Unlike gas key sealing, gas key staking is obvious to the casual user, so most manufacturers do it.

For more information about staking, check out our What’s at Stake article.

Proprietary Materials

Lewis Machine and Tool (LMT) has created a bolt using a proprietary alloy. They claim that this alloy is stronger and lasts longer than standard materials like C158 and 9310. For now, we will take their word for it.

The bolt has some other enhancements, but the proprietary unobtainium is the main selling point. The funny thing about secret formulas is that you can’t verify claims without empirical side-by-side testing with a known comparator. If they were smart, LMT would patent their formulation — and they may have (we just can’t find it); then they could be a little more transparent. One of these days, we will buy one to send out for ICP-MS, XRF, or some other analytical test to see if we can figure out what the material is. Then we compare it to the mechanical properties of its non-proprietary analog and estimate things like yield strength, ultimate strength, ductility, fatigue life, etc. For now, we accept that the formulation is a secret. Shh…

The problem we have with this product (other than the fact that it often goes out of stock for several months at a time) is that the bolt — by itself — costs $436; 4+ times more than a typical complete bolt carrier group. We find it hard to believe that this bolt is that special.

If you believe the hype, find it in stock, and have money to burn, go for it.

Enhanced Features

A few manufacturers have taken liberties with the design of the carrier and bolt. Some of these are legitimate enhancements. Some are compensating for a deficiency. Some are just a differentiator.

Optimized Carrier Key Screws (O.C.K.S.)

The Michiguns Optimized Carrier Key Screw (O.C.K.S.) is one of the few enhancements that offer a legitimate, tangible improvement over the Mil-Spec BCG.

The gas key screw has a critical job: it holds the gas key to the carrier. It bears a ton of force from the gas system, so it needs to be strong. The gas key needs to stay in place, no matter what, so the gas key screws need to be secure. If the gas key screws loosen or back out, catastrophe will ensue.

The O.C.K.S. are an improvement over the standard grade 8 gas key screws. Instead of just having a knurled side, O.C.K.S. have a castle profile cut into the edge of the screw head. When the gas key screw is staked, the material from the gas key has a place to go. These notches ensure excellent engagement between the gas key stake and gas key screw.

O.C.K.S. come standard in PBA BCGs, because they add tremendous value and reliability to the BCG.

Cam Track

The cam track in the bolt carrier is a unique area for enhancement. The theory goes that by playing with the geometry of the shape and length of the track, you can affect the “locked bolt time”.

Lantac did some interesting testing (found HERE) of longer cam track and determined that there is no positive effect. In fact, they determined that the carrier accelerates faster because of the delayed interaction with the cam pin. The manufacturers who use this design claim that it results in a more persuasive extraction. That is a “rose-colored” way of looking at it. The altered cam track results in higher velocity of the carrier when it actually engages the cam pin and momentum is proportional to mass and velocity, so they are technically correct. However, the more forceful interaction between the carrier and cam pin puts added stress and shock on the cam pin and bolt. Accordingly, Lantac rejected the altered cam pin track concept for their enhanced carriers.

The LMT Enhanced Carrier uses this altered cam track design.

Sand Cut Carrier

“Sand Cuts” are an interesting evolution to the bolt carrier. Sand cuts are angled cuts in the side rails of the carrier. The theory is that the interruptions of the carrier rails allow the carrier to push dirt and grit out of the way. This is absolutely a true statement. However it is only part of the story.

As much as these sand cuts allow dirt a grit to be pushed out of the way of the rails, they also give dirt and grit a clear path into the receiver. Anything that makes its way in through the ejection port will be funneled into the receivers.

We don’t use sand cut carriers. If you want them, there are a few manufacturers that offer them: Centurian Arms, Knights Armament, and KAK Industry are probably the best known.

Carrier Vents

Some manufacturers play around with the number and quantity of gas vents in the right side of the carrier. This can modulate the depressurization and venting angles. We don’t see anything wrong with these enhancements, as long as they don’t interfere with the normal operation of the gas system.

Bolt Lug Profile

Some manufacturers play around with the geometry of the bolt lugs.

LMT uses a cloverleaf side profile. They claim this relieves internal stresses and increases durability. That may be true. It may also be necessary since they use a non-conformant metal for the bolt and maybe it didn’t behave the way a TDP bolt does. Who knows.
Sharpe’s Rifle Company uses a right trapezoid profile on their Relia-Bolt^®. They claim that this shape “cuts through the crud”. The reality is that their bolts are made from S7 and were damaging the barrel extension, so they modified the profile of the lugs.

A Touch of Humility

Eugene Stoner’s final M16 bolt carrier group design, as immortalized in the TDP, is a stunning feat of engineering.

Don’t get us wrong…we appreciate innovation. We thrive on it. But if it ain’t broke, ain’t nothin’ to fix.

Many of the innovations bubbling up in the civilian market are gimmicky and intended to differentiate the manufacturer from the pack. Some of these ideas are valid. Most of them are not.

The M16/M4 BCG has been in service, as specified, for 50 years. If there was a better design, the U.S. Military would have figured it out by now and would have made the change.

Food for thought.